Atomic layer deposition (ALD) is a method of depositing a thin film on a substrate in very controlled manner. The deposition process is controlled by using two or more chemicals (“precursors”) in vapor form and reacting them sequentially and in a self-limiting manner on the substrate surface. The sequential process is repeated to build up the thin film layer by layer, wherein the layers are atomic scale.
ALD is used to form a wide variety of films, such as binary, ternary and quaternary oxides for advanced gate and capacitor dielectrics, as well as metal-based compounds for interconnect barriers and capacitor electrodes. An overview of the ALD process is presented in the article by George, entitled “Atomic Layer Deposition: an Overview,” Chem. Rev. 2010, 110, pp 111-113 (published on the Web on Nov. 20, 2009).
One type of ALD is called radical enhanced ALD (RE-ALD). RE-ALD utilizes radicals generated by a plasma to form one of the precursor gases. Because a radicalized precursor tends to be more reactive than its unradicalized counterpart, it helps induce the reactions when forming the film layers.
The type of plasma used in RE-ALD is referred to by the name of the feedgas used to form the plasma. For example, an “oxygen plasma” utilizes oxygen (O2) as the feedgas to produce a plasma that generates oxygen-radicals precursors. The oxygen-radical precursors comprise monatomic oxygen, which has two unpaired electrons that make monatomic oxygen very reactive. The oxygen radicals serve as co-reactants that are used in conjunction with a second precursor (say, Si). The two precursors are feed sequentially into the reactor chamber to produce sequential layers that lead to thin film growth (e.g., SiO2).
While ALD has many advantages, one of its major disadvantages as compared to other film growth processes (such as chemical-vapor deposition or CVD) is that it is quite slow. For example, a conventional ALD reactor has a growth rates measured in angstroms/minute, while CVD has growth rates measured in microns/minute. Slow growth rates result in excellent film quality but limit the throughput of processed substrates (wafers) in a semiconductor manufacturing line. Thus, while an oxygen plasma is effective in providing oxygen-radical precursors for oxygen-based RE-ALD, there is always a need to increase the speed and/or efficiency of an oxygen-based RE-ALD process.